Now before I answer this question,
let's make it clear that it's not necessarily any of your business.
Some women don't mind telling you they're in their period if you ask
politely. Others do mind.

Either
way, remember this: If a woman is in a bad mood and complaining about
you, she probably has a good reason, regardless of what time of the
month it is. Your best bet is to shut up and do something
spontaneously nice for her. Everyone is happier this way.

But
to answer the question, here are some simple signs you can look for:

Consumption of
chocolate

Holding hips as
if in pain

Legs tightly
crossed when sitting

Acting bitchy

Do
not, however, under any circumstances, try to find out yourself by
investigating. If you stick your head up her
dress, chances are she will remove your balls and shove them down
your throat. Have a nice day.

Note from the Male Editor: She
didn't remove my balls, she just stepped on them. It was a big
mistake on my part to lay down under a woman wearing high-heels...